Connections between manifold/header plate and tubes in heat exchangers employed, e.g. as condensers or radiators in a vehicle, are in principle provided by two methods, i.e. soldering/brazing or mechanical expansion of the tubes after their insertion into the manifold or header plate. All methods are at the present time characterized by several problems related to the providing of a reliable, leakage-proof connection having satisfactory mechanical strength.
GB Pat. No. 1,492,555 describes a heat exchanger for vehicles based upon a mere expansion connection between the tubes and the manifold without use of supplementary rubber packing. The construction is characterized by a precisely specified interaction between a number of parameters such as wall thickness, tube diameter, material strength, length of support and others expressed by empirical formulas. This construction requires high tolerance from the components, and furthermore it has obvious limitations with regard to free choice of materials, wall thickness, tube diameter, etc.
A further disadvantage of the disclosed construction is represented by the process employed for making fastening apertures in the tube wall. The wall is "knocked down" and pressed and drawn further inwards to form a collar or flange which ensures the necessary support or backing area for fastening of a heat exchanger tube. The height of the collar is related to the wall thickness of the manifold, which gives poor possibilities of achieving an adequate collar height on thin-walled manifolds due to the limited material mass which can be transferred in the deformation zone.
Limited material flow is also the disadvantage of the so-called T-drill or Flow-drill processes, where by means of special tools the manifold tube is perforated and a low collar is formed. Depending on the type of tool employed, this collar is formed on the outside or both of the innerside and on the outside of the manifold. In those cases where the collar protrudes inwards into the manifold the available, free cross-section of the manifold is reduced and an increased pressure drop in the heat exchanger arises due to turbulent currents in the manifold.
Additionally to the above mentioned disadvantages also several other problems arise by brazing of the heat exchanger tubes to such manifolds. Because of the short collar a penetration of brazing material and flux along the tubes into the manifold occurs quite frequently, which further contributes to the reduction of the available, free cross-section. Flux residues being entrapped in this way are difficult to remove and they have a corrosion promoting effect on the components. Furthermore, it is difficult to achieve a tight and rigid connection because of the relatively short available brazing length. In the case of inwards protruding brazing hollow risers it is difficult to control the amount of heat since the joining zone is concealed in the manifold. Differences in wall thickness between the joined components represents another obstacle to achieving a proper control over heat balance in the joining process. Overheating of the tubes and mechanical weakening of the connection as a result of this, therefore represents an imminent danger of a high reject rate by manufacture of heat exchangers according to this principle.
Co-pending U.S. patent application Ser. No. 793,285 filed Oct. 31, 1985 discloses still another manifold construction and method of its manufacture. The construction is characterized by provision of a shaped tube with an outwardly, longitudinally extending protrusion which forms an integral portion of fastening flanges for the heat exchanging tubes. The resulting manifolds overcome the above mentioned disadvantages, ensuring leakage-proof and rigid connection to the heat exchanger tubes. However, the disclosed manufacturing method and resulting manifold design do not offer a flexible solution with regard to current, alternative methods of joining manifolds to heat exchanger tubes. This manifold design, for example, allows only for insertion of tubes into the fastening flanges so that not all presently installed industrial assembling lines can be used for the subsequent brazing process.
There is still another known process of manifold manufacture where hollow risers are provided by a step wise deep drawing operation performed on aluminum sheet material. The reworking steps in this process are bending of the sheet and welding of butted sheet ends in order to form a tube. A weakness of this construction is a longitudinally running welded seam which does not provide a reliable and tight rigid joint under high pressure in heat exchangers. The thickness of the applied sheet limits also in this case the achieved maximum length of the risers.